1. Field of the Invention
The present invention is related to the field of manual lever hoists. More specifically, the invention relates to overload protection devices for levers which limit the amount of torque which can be applied to the hoist.
2. Description of Related Art
Various manual lever hoists have been generally known in the art and are commonly used to apply or release tension to a chain or a cable. For instance, such manual lever hoists are commonly used in industry to tighten cables, lift objects as well as various other material handling purposes. Examples of manual lever hoists used in lifting applications are generally shown and taught in various prior art documents such as U.S. Pat. No. 4,325,470 to Bopp, U.S. Pat. No. 4,768,754 to Nishimura, and U.S. Pat. No. 5,791,579 to Raphael et al. These manual lever hoists include a hoist and a lever attached thereto for operating the hoist. The hoist typically engages a chain (or a cable) and are operable to displace the chain so as to lift or lower an object removably attached thereto. In this regard, the manual levers used in conjunction with these hoists generally have a ratchet assembly. The hoists can be manually operated via the ratchet assembly to displace the chain so that lifting or lowering of objects can be attained by repeatedly pivoting the lever. The length of the lever provides mechanical leverage to exert high torque on the hoist so that heavy objects can be lifted.
Some models of levers also have a ratchet assembly with an overload protection device that essentially act as a clutch so that only a predetermined amount of torque may be applied to the hoist via the lever. Such an overload device feature is important in many applications of the manual lever hoists because it prevents the overloading of the hoist, chain, and/or any other component such as the hook attached to the chain or the structure supporting the hoist. If the hoist, chain and/or other components are overloaded such as in a case where the object being lifted weighs more than the rated capacity of the hoist, chain and/or other component, there is a danger of one or more of the components failing and thus, damaging the hoist. Such failure of a component can cause the object being lifted to be dropped and damaged as well. More importantly, the falling object can also cause great bodily injury or even death to anyone in the object""s path.
U.S. Pat. No. 4,325,470 to Bopp discloses an overload clutch having a plurality of first clutch disks which frictionally engage a plurality of second clutch disks to thereby transmit torque from the lever to the hoist. The overload clutch is preloaded to a torque capacity corresponding to the rated load of the hoist such that the clutch disks will slip relative to one another when the torque capacity of the overload clutch is exceeded. The overload clutch disclosed in Bopp however, requires large number of components and is very complicated and difficult to manufacture and assemble. As a result, the overload clutch disclosed in Bopp is also expensive and does not provide an economically practical overload clutch.
U.S. Pat. No. 5,791,579 to Raphael et al. discloses another overload prevention clutch assembly which includes a resiliently deformable coupling member which is controlled by a threaded fastener to thereby vary the torque which may be transmitted between the lever and the hoist. While requiring less components than the clutch of Bopp, the overload prevention clutch assembly of Raphael et al. however, is also relatively complicated and is difficult to manufacture and assemble. As a result, the overload prevention clutch assembly disclosed in Raphael et al. is also expensive and does not provide an economically practical overload prevention assembly.
U.S. Pat. No. 4,768,754 to Nishimura discloses yet another overload preventor for use with a manual hoist which includes a drive gear with teeth around its periphery, a force exerting member, and a conical friction ring disposed therein between for frictionally engaging the drive gear with the force exerting member. The torque capacity of the overload preventor is determined by the pressing force exerted on the conical friction ring and is adjusted by a nut threaded on to the force exerting member. To ensure proper engagement between the conical friction ring and the force exerting member, the overload preventor disclosed in Nishimura requires that the conical friction ring be provided with projections which engage recesses of the force exerting member. Such required provisions are difficult to manufacture add to the component manufacturing costs, thereby increase the cost of the overload preventor. Moreover, to ensure proper pressing force, the overload preventor disclosed in Nishimura requires various washers and fasteners. Such additional components further increase the cost of the overload preventor and also increases the cost associated with assembling such additional components.
Therefore, in view of the above noted deficiencies of the prior art devices, there exists an unfulfilled need for a simple overload protection device for a lever hoist which minimizes the number of components required and the complexity of such components to thereby minimize the associated component costs. In addition, an unfulfilled need still exists for such an improved overload protection device which is easy to assemble so as to minimize assembly costs.
In view of the foregoing, it is an object of the present invention to provide an improved overload protection device for a lever of a manual lever hoist which minimizes number and the complexity of the components required.
A second object of the present invention is to provide such an improved overload protection device which is easy to manufacture and assemble.
A third object of the present invention is to provide such an improved overload protection device which can be adjusted for use in various manual lever hoist applications.
Yet another object of the present invention is to provide an improved overload protection device which is economical.
In accordance with one preferred embodiment of the present invention, these objects are obtained by an improved overload protection device for a manual hoist lever including a pressure hub, an outer hub, a brake nut, and a dowel pin. In accordance with the preferred embodiments, the pressure hub includes a radially extending flange and a substantially centrally positioned cantilevered boss, the boss having a threaded outer surface with an axial groove extending from an end of the boss toward the radially extending flange. The outer hub includes a central opening, a tapered inner surface, and a flange contact surface which frictionally contacts the radially extending flange of the pressure hub. The brake nut is disposed in the central opening of the outer hub between the outer hub and the boss of the pressure hub. The brake nut also includes a frusto-conical outer surface that frictionally contacts the tapered inner surface of the outer hub and a threaded inner opening with a corresponding axial groove, the threaded inner opening being sized to threadingly engage the threaded outer surface of the boss. The dowel pin is received within a receiving hole formed upon alignment of the axial groove of the boss and the corresponding axial groove of the brake nut, thereby locking the pressure hub and the brake nut together to prevent relative rotation thereof. In this regard, the axial groove of the pressure hub and the corresponding axial groove of the brake nut are preferably semi-circular so that a circular hole is formed upon alignment of the axial groove and the corresponding axial groove, and a cylindrical dowel pin is received therein.
In the preferred embodiment of the overload protection device in accordance with the present invention, the pressure hub is adapted to be attached to a hoist and the outer hub is adapted to be engaged by a lever for operating the hoist. The outer hub preferably includes a plurality of ratchet gear teeth positioned on an outer surface of the outer hub, the plurality of ratchet gear teeth being adapted to be engaged by a tapered rod end or a ratchet pawl of the lever. The pressure hub preferably also includes a threaded attachment hole extending through the boss for receiving a threaded end of a pinion shaft of the hoist. The outer hub is rotatable relative to the pressure hub and the brake nut during an overload condition which occurs when a relative rotational force overcomes frictional resistance of the overload protection device. The frictional resistance is caused by frictional contact between the flange contact surface of the outer hub and the radially extending flange of the pressure hub, and also by the frictional contact between the tapered inner surface of the outer hub and the frusto-conical outer surface of the brake nut. In this regard, in the preferred embodiment, the radially extending flange extends substantially normal to the boss, and the flange contact surface of the outer hub flushly contacts the radially extend flange to thereby generate frictional resistance to relative rotation between the pressure hub and the outer hub. In addition, the tapered inner surface of the outer hub flushly contacts the frusto-conical outer surface of the brake nut to thereby generate frictional resistance to relative rotation between the outer hub and the brake nut.
The frictional resistance of the overload protection device is at least partially determined by the axial positioning of the brake nut relative to the radially extending flange and is preferably adjustable by tightening or loosening the brake nut. For instance, in one embodiment, the tightening of the brake nut increases the frictional resistance of the overload protection device while loosening the brake nut decreases the frictional resistance of the overload protection device. In this regard, the brake nut preferably includes an engagement opening adapted to allow receipt of a tool for threadingly engaging and adjusting the brake nut on the threaded outer surface of the boss. The engagement opening preferably has a hexagonal shape or a rectangular shape. In addition, to assist in the assembly of the overload protection device, the radially extending flange of the pressure hub may also include a plurality of locating holes.
In another embodiment of the overload protection device, the outer hub is adapted to be attached to a hoist and at least one of the pressure hub and the brake nut is adapted to be engaged by a lever. In such an embodiment, the pressure hub includes a plurality of ratchet gear teeth positioned on a peripheral edge surface of the radially extending flange, the plurality of ratchet gear teeth being adapted to engage a tapered rod end or a ratchet pawl of the lever.
These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the invention when viewed in conjunction with the accompanying drawings.